The invention relates to the production of tertiary carboxylic acids (also called neoacids) from carbon monoxide, the appropriate olefins, and water on an acid catalyst and, in particular, to the purification of the resultant products.
Tertiary carboxylic acids can be produced according to the so-called "Koch synthesis" from the appropriate olefins, carbon monoxide, and water. Thus, e.g., 2,2-dimethyl propanoic acid (trimethylacetic acid, pivalic acid) is obtained from isobutene: ##STR1##
Mainly used as catalysts are strong protonic acids, such as, e.g., sulfuric acid, phosphoric acid, and hydrofluoric acid, or Lewis acids, such as, e.g., boron trifluoride, aluminum chloride, and antimony (V) chloride [Houben-Weyl, Methoden der organischen Chemie (Methods of Organic Chemistry), Fourth Edition, Vol. E5, 1985, p. 315).
In separating the byproducts resulting from the "Koch synthesis", it is known that prior to the distillation of the tertiary carboxylic acids, the reaction mixture can be washed with water (Hydrocarbon Processing, Vol. 43, No. 11, November 1964, pp. 186-187), as well as with sulfuric acid, sodium bicarbonate, and citric acid [Ullmanns Encyklopaedie der technischen Chemie (Ullmann's Encyclopedia of Industrial Chemistry), Fourth Edition, Vol. 9, p. 40, and J. Falbe, "New Synthesis with Carbon Monoxide, 1980, p. 407].
According to JP-AS 73/16897, the crude product containing the tertiary carboxylic acids is treated before distillation with an oxidizing agent, such as, e.g., air or oxygen, and, according to U.S. Department of Defense Publication No. 864,004, is catalytically hydrogenated.
DE-OS 15 68 391 teaches a process for the production of neoacids according to the "Koch synthesis", and the formed neoacids are obtained by extraction with cyclohexane and aqueous potassium hydroxide from the reaction product (see Example 2).
Also, according to U.S. Pat. Nos. 3,489,779 and 3,151,139, the formed neoacids are separated from the reaction mixture by extraction; namely, with heptane as the extraction agent or with an aqueous ammonia solution in the presence of a water-immiscible organic solvent which retains the impurities.
The processes according to the prior art for obtaining tertiary carboxylic acids from the crude product of the "Koch synthesis" are thus complex, expensive, and difficult to perform, especially in liquid-liquid extractions, where small amounts of secondary ingredients or impurities particularly hamper operations.
JP-AS 71/35724 discloses a process for obtaining trimethylacetic acid, in which dimeric and trimeric isobutene are distilled as azeotropic mixtures with water and n-amyl alcohol from the crude product obtained in the "Koch synthesis".
According to JP-AS 73/00807, the trimethylacetic acid crude product of the "Koch synthesis" is subjected to azeotropic distillation to remove as impurities dimeric isobutene with water as entrainer and trimeric isobutene with water and trimethylacetic acid as an entrainer. ("Entrainer" is synonymous with azeotrope-former and entraining agent.)
The working-up processes of JP-AS 71/35725 and JP-AS 73/00807 are suitable solely for the isolation of trimethylacetic acid, and only dimeric and trimeric isobutene can be azeotropically distilled off as impurities from the trimethylacetic acid crude product.
JP-AS 73/23412 discloses a process for obtaining tertiary nonanoic acids. In this case, tetrameric isobutene with diethylene glycol ether as the entrainer is separated by azeotropic distillation from the crude product obtained in the "Koch synthesis". This working-up process is suitable exclusively for isolating tertiary nonanoic acids, and basically only tetrameric isobutene can be separated from the impurities by distillation.